I think I will wait till jailbreak of 5.3.0, usually require lot of time like the ios one?

It depends. For the older versions, bugs that were exploitable to allow for a jailbreak were relatively easy to find, but Amazon gradually fixed all of them.

I have been searching for exploits for two weeks now, and I did not find anything that can be used for jailbreaking 5.3.0. I officially give up now, maybe someone else is better/luckier than I am at this.

Quote:

How can I prevent the autoupgrade if another firmware come out? Only with airplane mode?

Airplane mode is the easiest measure, but you could also selectively disable or remove part of the programs that play together for the OTA update (No, I can't tell you which ones these are off the top of my head).

I have been searching for exploits for two weeks now, and I did not find anything that can be used for jailbreaking 5.3.0. I officially give up now, maybe someone else is better/luckier than I am at this.

I'd like to say: you've made a great work in tries for finding jailbreak vector. Thanks for your investigations, it was interesting to follow them!

is it possible for a firmware/OS to be so "well" written that it can't be jailbroken?

In principle, yes - even if that is *very* difficult. Essentially, it would mean that there is absolutely no "exploitable" bug anywhere, in any of the code.

I'm actually convinced that such a bug exists, somewhere, also in 5.3.0. The difficulty lies in actually finding it. Mind you, I did find multiple different bugs of varying severity* in 5.3.0, but none of them is severe enough to be used as a jailbreak vector.

(*) No, I'm not going to disclose them publicly. But don't worry: these things are being discussed internally, in a small circle of expert developers.

is it possible for a firmware/OS to be so "well" written that it can't be jailbroken?

In the case of Unix and its derivative and "work a likes" (such as Linux), a qualified: YES.

The qualification:
No system can be secure if the system's operator console is accessible.

In this case, see the indexed information on accessing the serial port (which becomes the system's operator console once Linux boots).

- - - -

The above is the general answer, a more specific answer - - -

Here (at Mobileread) the "jailbreak" method(s) are expected to be run by any end-user, regardless of knowledge or skill levels.

Such a qualification eliminates the use of the serial console.
With this additional qualification, the answer is: "Yes, It can be locked from end-user alteration".

In fact, without even trying very hard.

As a contrast, see how many success stories you can find of anyone "jailbreaking" the OS on the Qualcom SoC that runs your 3G card.

It is also a signed binary, only in that case the signature verification is done in hardware buried inside the chip.
Something the Kindle SoC could also do, but Amazon does not use that feature. Which is what I meant above by "without even trying very hard".

It depends. For the older versions, bugs that were exploitable to allow for a jailbreak were relatively easy to find, but Amazon gradually fixed all of them.

I have been searching for exploits for two weeks now, and I did not find anything that can be used for jailbreaking 5.3.0. I officially give up now, maybe someone else is better/luckier than I am at this.

Airplane mode is the easiest measure, but you could also selectively disable or remove part of the programs that play together for the OTA update (No, I can't tell you which ones these are off the top of my head).